(1) Field of the Invention
The present invention relates to the technical field of the flight mechanics of a rotary wing aircraft. It relates to a method of controlling such an aircraft during a stabilized stage of flight at high speed. It also relates to a control device for such an aircraft.
The method and the device are intended more particularly for hybrid helicopters, i.e. rotary wing aircraft fitted with auxiliary propulsion.
(2) Description of Related Art
A rotary wing aircraft conventionally comprises at least one main rotor, serving to provide the aircraft both with lift and with propulsion, a fuselage, and a power plant.
A hybrid helicopter also includes at least one propulsive propeller and a lift-providing surface, or more simply a “lift surface”, generally made up both of two half-wings situated on either side of the fuselage and of a horizontal stabilizer positioned at one end of the aircraft.
By way of example, two variable-pitch propulsive propellers are positioned on either side of the fuselage, one on each of the half-wings.
In addition, each half-wing may be fitted with at least one movable flap enabling the lift of each half-wing to be modified. Likewise, the horizontal stabilizer includes at least one movable surface in order to modify the lift of the horizontal stabilizer. The movable surface may be constituted by an elevator surface or by the horizontal stabilizer as a whole.
The main function of the two half-wings is to contribute to providing the hybrid helicopter with lift while it is in flight and flying at a high forward speed, with the propulsive propeller(s) making it possible to reach such a speed. In contrast, it is possible to speak of flight at moderate speed for flights at forward speeds that are slower than that of a cruising flight.
When flying at high forward speed, the movable horizontal stabilizer or the elevator surface of the hybrid helicopter are equivalent to a trim compensator in an airplane. Pitching control of the hybrid helicopter is performed using the cyclic control of the main rotor, while the movable horizontal stabilizer (or its elevator surface) serves to adjust the pitching equilibrium point of the aircraft in application of various criteria such as the attitude of the aircraft, or indeed the bending moment of the mast of the main rotor. In such a configuration, the half-wings contribute to the total lift of the aircraft needed for keeping the aircraft in the air. Consequently, the main rotor provides part of the lift in a hybrid helicopter when flying with high-speed forward speed, and possibly also contributes to propelling it forwards.
It can thus be seen that piloting a hybrid helicopter during high-speed forward flight requires special controls in order to modify the lifts of the half-wings and of the horizontal stabilizer, and also the pitches of the propulsive propellers.
Consequently, during flight at a high forward speed the workload on the pilot is large and complex in order to be able to manage the specific controls of the hybrid helicopter in addition to the conventional controls of a rotary wing aircraft.
An object of the present invention is thus to provide a method of assisting the pilot in order to determine and adjust the lifts of the half-wings and of the horizontal stabilizer specific to such a hybrid helicopter during a stabilized stage of flight.
The term “stabilized stage of flight” is used to mean flight at high forward speed, and thus applies to hybrid helicopters when flying conditions are constant, i.e. when the main flight parameters are constant. This applies in particular to the vertical speed and to the path of the aircraft. Constant vertical speed may be obtained in particular by maintaining the attitude and/or the angle of incidence of the aircraft constant. In the special situation where the vertical speed is zero, the stabilized stage of flight takes place at constant altitude, and can then be referred to as “cruising” flight. A constant path corresponds to a path without the aircraft changing heading. During such a stabilized stage of flight, the forward speed is preferably also constant. Nevertheless, it may vary, but generally slowly. In the method of the invention, dynamic variations take place slowly, so the method is compatible with forward speed varying in such a manner.
Document US 2008/0237392 describes a hybrid helicopter using a control system for controlling all of the controls of the aircraft. The aircraft incorporates databases of optimized and predefined flight parameters for different flight conditions and different types of flight.
The pilot selects the type of flight that is to be performed, e.g. minimizing fuel consumption, minimizing vibration, or indeed maximizing forward speed. The control system determines flight conditions by using various sensors incorporated in the hybrid helicopter and then selects from the database various predefined settings for the various controls of the aircraft corresponding to such flight conditions.
The control system then communicates these predetermined settings to the autopilot which acts without intervention from the pilot to apply them to the various control members of the hybrid helicopter.
Document FR 2 959 205 describes a method of controlling and regulating the deflection angle of a horizontal stabilizer of a hybrid helicopter at a stabilized high speed of advance. The purpose of that adjustment of the deflection angle is to optimize the power consumed by the aircraft.
That method comprises three regulation loops. The first loop controls the attitude of the aircraft by means of the longitudinal cyclic pitch, and the second loop controls the forward speed of the aircraft by means of the pitch of the propulsive propellers. Those two loops ensure that the aircraft is stabilized in longitudinal attitude and in forward speed. Finally, the third loop optimizes the power of the aircraft by means of the deflection angle of the horizontal stabilizer while maintaining the longitudinal attitude and the forward speed constant.
Any variation in the deflection angle of the horizontal stabilizer modifies its lift. Consequently, since the longitudinal attitude of the aircraft is kept constant by the first regulation loop, such variation in the lift of the horizontal stabilizer serves to subject the fuselage to a pitching moment in a nose-down or a nose-up direction. It is then appropriate to act on the orientation of the main rotor so that it tends towards a “nose-up” attitude or a “nose-down” attitude as required in order to counter the effect of the pitching moment of the stabilizer.
When the main rotor tends towards a nose-down attitude, it provides propulsion, i.e. it contributes to making the aircraft advance, providing it is being driven in rotation by the power plant of the aircraft. In contrast, when the main rotor is tending towards a nose-up attitude, it is in autogyro mode, i.e. it is not driven in rotation by the power plant, but rather by the flow of air created by the aircraft moving forwards. Under such circumstances, the main rotor serves to generate only lift.
Consequently, modifying the angle of deflection of the horizontal stabilizer has an effect on the operation of the main rotor and in particular on the power it consumes.
Furthermore, document FR 2 916 420 describes a hybrid helicopter having at least one elevator surface on a horizontal stabilizer with a deflection angle that can be controlled as a function of the bending moment of the mast of the main rotor. In addition, the cyclic pitch control for the blades of the main rotor enables the longitudinal attitude of the hybrid helicopter to be controlled, and the lift of the wings of that hybrid helicopter may thus be set to some particular percentage of the total lift in cruising flight.
In addition, document WO 2005/005250 describes a hybrid helicopter in which the wings provide about 70% of the total lift during cruising flight.
Also known is document FR 2 916 419, which describes a hybrid helicopter in which the speed of rotation of the main rotor can be reduced in cruising flight. Controlling the longitudinal cyclic pitch of the blades of the main rotor then enables the drag of the fuselage of the hybrid helicopter to be reduced. In addition, that helicopter has at least one elevator surface on a horizontal stabilizer with a deflection angle that can be controlled in order to cancel the bending moment of the mast of the main rotor.
Also known is an autopilot device for a hybrid helicopter that enables the aerodynamic angle of incidence of the aircraft to be kept constant and equal to a reference angle of incidence while performing stabilized cruising flight. In order to maintain this angle of incidence constant, the autopilot acts on the collective pitch of the blades of the main rotor.
Likewise, that device makes it possible to maintain the longitudinal attitude of the aircraft about its pitching axis constant and equal to a reference attitude while performing stabilized cruising flight. Under such circumstances, the autopilot acts on the longitudinal cyclic pitch of the blades of the main rotor.
In addition, the device provides a display that may show the reference angle of incidence and the reference attitude. The pilot can then see on the display both the actual attitude and angle of incidence of the aircraft and, where appropriate the corresponding reference values.